Project Summary We have previously reported that metastatic melanoma cells and tumor specimens display increased activation of CREB. CREB serves as a survival factor for melanoma cells and contributes to melanoma metastasis by regulating several genes including MCAM/MUC18, MMP-2, MITF, and CYR-61. CREB is also responsible for the loss of AP-2?, thus making it a master regulator in melanoma progression. This application is based on our recent observation that CREB also regulates the expression of adenosine deaminase acting on RNA (ADAR1) enzyme. Adenosine to Inosine (A-to-I) editing is the most prevalent form of mRNA and microRNA editing. Here, we present evidence of a CREB-dependent hypo-editing and low expression of ADAR1 in metastatic melanoma cell lines and tumor specimens. The mechanistic involvement of ADAR1 mediated microRNA editing and its role in melanoma metastasis has never been studied before. Our central working hypothesis is that ADAR1 is involved in the biogenesis and function of microRNA, which in turn plays a role in regulating genes involved in melanoma progression. This grant application is based on our recent observation published in Nature Cell Biology (2015), in which we identified 3 miRs (miR-455-5p, miR-378-3p and miR-324-5p) undergoing A-to-I editing in the non-metastatic melanoma cells, but not in the metastatic cells that do not express ADAR1. One of these miRs, miR-455-5p has two A-to-I RNA editing sites. The function of the edited miR is different from the unedited form, as it recognizes different sets of genes involved in melanoma progression. Indeed, WT (but not the edited form) of miR-455-5p preferentially binds to the 3? UTR of the tumor suppressor gene CPEB1 and inhibits its expression. In addition, wild type miR-455-5p enhances melanoma growth and metastasis while the edited form inhibits these features. The proposed studies are aimed to fulfill this gap in our understanding of how certain miRNAs undergoing A-to-I editing contribute to melanoma growth and metastasis. This is a new direction in melanoma research. Our studies could also identify new targets and treatment modalities.